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EIGRP Tables

EIGRP Tables

Three EIGRP Tables are maintained by EIGRP to execute its operation i.e. Neighbor Table, Topology Table, and IP Routing table for its operation. This article will discuss these three tables in detail.

EIGRP Tables

How EIGRP selects Best Routes?


Step 1 - Establish an adjacency or neighbor-ship (Neighbor Table) with other peers or devices running EIGRP and have a common link subnet.

Step 2 - Store all the routes in the Topology table. Also, exchange the EIGRP topology database (Topology) Table with neighbors formed in step1.

Step 3 - Calculate the best route/path for each subnet present in the Topology Table and install them in the Routing Table.

Let's discuss each of 3 tables in detail using below topology.

 

EIGRP Tables

 

EIGRP Neighbor Table

It is the first step of EIGRP operation to discover the neighbors. A router running EIGRP multicasts Hello packets to discover the neighbors. An adjacency is created with these neighbors so that it can exchange route updates as a second step. Only the adjacent routers exchange routing information. Each router builds a neighbor table from the Hello packets it receives from adjacent EIGRP routers. To check the neighbor table, you can run "show ip eigrp neighbors" command.

R1#show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(10)
H   Address                 Interface       Hold   Uptime   SRTT      RTO  Q    Seq
                                                       (sec)                   (ms)                Cnt  Num
2   10.10.13.3              PO2/0            10   00:01:05  369       2214   0      6
1   10.10.12.2              Fa1/0             12   00:01:07  256       1536   0     11
0   10.10.14.4              Se3/0             12   00:01:07  276       1656   0     15

 

EIGRP Neighbor Table Fields Explanation

  • H (handle) - A number used internally by the Cisco IOS to track a neighbor. If there is a fourth router connected to R1, it would have a Handle number of 3.
  • Address - The neighbor’s IP address. It is the interface address of the connected interface of neighbors.
  • Interface: The next-hop interface on this router through which the neighbor can be reached.
  • Hold Time - The maximum time, in seconds, that the router waits to hear from the neighbor without receiving anything from a neighbor before considering the link unavailable.
  • Uptime - The elapsed time, in hours, minutes, and seconds since the local router first heard from this neighbor.
  • Smooth Round Trip Timer (SRTT) - The average number (ms) it takes for an EIGRP packet to be sent to this neighbor and for the local router to receive an acknowledgment of that packet. This timer is used to determine the retransmit interval, also known as the retransmit timeout (RTO).
  • RTO - The amount of time, in milliseconds, that the router waits for an acknowledgment before retransmitting a reliable packet from the retransmission queue to a neighbor.
  • Queue count - The number of packets waiting in the queue to be sent out. If this value is constantly higher than 0, a congestion problem might exist. A  value of  '0' indicates that no EIGRP packets are in the queue. If the value of Q Count is non zero, there is a delay, jitter on the link connected between neighbors and there will be slow convergence issues observed.
  • Seq Num - The sequence number of the last update, query, or reply packet that was received from this neighbor.


EIGRP Topology Table

EIGRP Topology Table is a kind of control plane or a database of EIGRP where all routes learned from a neighbor are initially stored before the best routes are moved to the routing table.

We can use the below commands to check the details of the topology table.

1.     show ip eigrp topology
2.     show ip eigrp topology all-links

The first command shows only the details of the successor as well as feasible successor links. We can see from the below output that for prefix 172.10.0.5/32, there is only next hop which is of successor (best path to reach 172.10.0.5/32). For the destination prefix 10.10.45.0/24, there are 2 best paths (equal-cost) and one FS.

R1#show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(10)/ID(172.10.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status


P 172.10.0.5/32, 1 successors, FD is 149504
        via 10.10.13.3 (149504/146944), POS2/0
P 172.10.0.3/32, 1 successors, FD is 146944
        via 10.10.13.3 (146944/128256), POS2/0
P 10.10.13.0/24, 1 successors, FD is 18944
        via Connected, POS2/0

P 10.10.45.0/24, 2 successors, FD is 2174976
        via 10.10.12.2 (2174976/2172416), FastEthernet1/0
        via 10.10.13.3 (2174976/2172416), POS2/0
        via 10.10.14.4 (2681856/2169856), Serial3/0

----------------output omitted------------------

The second command slows all links irrespective of a path is a successor or a  feasible successor or not, as shown below.

R1#show ip eigrp topology all-links
EIGRP-IPv4 Topology Table for AS(10)/ID(172.10.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 172.10.0.5/32, 1 successors, FD is 149504, serno 19
        via 10.10.13.3 (149504/146944), POS2/0
        via 10.10.12.2 (158720/156160), FastEthernet1/0
        via 10.10.14.4 (2809856/2297856), Serial3/0
P 172.10.0.3/32, 1 successors, FD is 146944, serno 17
        via 10.10.13.3 (146944/128256), POS2/0
P 10.10.13.0/24, 1 successors, FD is 18944, serno 4
        via Connected, POS2/0
P 10.10.45.0/24, 2 successors, FD is 2174976
        via 10.10.12.2 (2174976/2172416), FastEthernet1/0
        via 10.10.13.3 (2174976/2172416), POS2/0
        via 10.10.14.4 (2681856/2169856), Serial3/0

----------------output omitted------------------



EIGRP Active Route vs EIGRP Passive Route

A route is considered to be an EIGRP Passive Route when the router is not performing any calculation or computation on that route. On the other hand, a route is considered to be in EIGRP Active Route when it is undergoing re-computation (for example, when it is looking for a new successor). A Passive route is the operational, stable state of the route.

If an FS (Feasible successor) for a prefix is always available, a destination prefix never has to go into the active state, thereby avoiding a re-computation. A prefix goes in Active state when the successor to a destination prefix, goes down and there is no FS available for the destination. The router initiates a re-computation by sending a query packet to each of its neighboring routers. If the neighboring router has a route for the destination, it will send a reply packet; if it does not have a route, it sends a query packet to its neighbors.


Advertised Distance

The Advertised Distance (AD) is equal to the cost of the path to the destination prefix as advertised by neighboring routers. For example, for the prefix 172.10.0.5/32 in the below output of the topology table, the value 146944 is AD.

P 172.10.0.5/32, 1 successors, FD is 149504, serno 19
        via 10.10.13.3 (149504/146944), POS2/0

 

Feasible Distance

Feasible Distance (FD) It is equal to the sum of the AD for a neighbor to reach the destination prefix plus the metric to reach that neighbor. For example, for the prefix 172.10.0.5/32 in the below output of the topology table(show ip eigrp topology), the value 149504 is FD.

P 172.10.0.5/32, 1 successors, FD is 149504, serno 19
        via 10.10.13.3 (149504/146944), POS2/0


Successor

The successor is the forwarding path (best route) used to reach the destination prefix. The cost of this path is equal to the FD. For example, for the destination prefix 10.10.45.0/24 from the topology table(show ip eigrp topology), there are 2 successors i.e. best equal-cost paths to the destination. The prefix also has an FS via 10.10.14.4.

P 10.10.45.0/24, 2 successors, FD is 2174976
        via 10.10.12.2 (2174976/2172416), FastEthernet1/0
        via 10.10.13.3 (2174976/2172416), POS2/0
        via 10.10.14.4 (2681856/2169856), Serial3/0


Feasible Successor

The Feasible Successor (FS) is an alternative loop-free path to reach the destination Prefix. For example, in the below output from the topology table(show ip eigrp topology), the last path is Feasible Successor (FS).

P 10.10.45.0/24, 2 successors, FD is 2174976
        via 10.10.12.2 (2174976/2172416), FastEthernet1/0
        via 10.10.13.3 (2174976/2172416), POS2/0
        via 10.10.14.4 (2681856/2169856), Serial3/0

 

Condition to become a Feasible Successor

The condition for a path to qualify as an Feasible Successor it that the next-hop router must have an AD less (strictly) than the FD of the current successor route for the particular prefix. Equal AD and FD values also don't meet the condition. For example, in the below output for prefix 10.10.0.45/24 from the topology table, the last path is FS because it qualifies the condition. AD value 2169856 is less than the FD of Successor paths (2174976).

P 10.10.45.0/24, 2 successors, FD is 2174976
        via 10.10.12.2 (2174976/2172416), FastEthernet1/0
        via 10.10.13.3 (2174976/2172416), POS2/0
        via 10.10.14.4 (2681856/2169856), Serial3/0

For prefix 172.10.0.5/32, there is only one Successor, but no Feasible Successors because the last 2 paths don't qualify the condition to become FS.  The AD values 156160 and 2297856 are higher compared to FD 149504 of the Successor path.

P 172.10.0.5/32, 1 successors, FD is 149504, serno 19
        via 10.10.13.3 (149504/146944), POS2/0
        via 10.10.12.2 (158720/156160), FastEthernet1/0
        via 10.10.14.4 (2809856/2297856), Serial3/0



EIGRP IP Routing Table

The best paths of the Topology Table are moved to the EIGRP IP Routing Table, which can be checked using "show ip route"  or to specifically check only EIGRP routes, use the "show ip route eigrp" command. The EIGRP routes are denoted by "D" with an Administrative Distance (AD)  value of 90.  The External EIGRP routes are denoted by "D-EX" and have an AD value of 170.

R1#show ip route eigrp
      10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
D        10.10.25.0/24 [90/30720] via 10.10.12.2, 02:46:30, FastEthernet1/0
D        10.10.35.0/24 [90/21504] via 10.10.13.3, 02:46:31, POS2/0
D        10.10.45.0/24 [90/2174976] via 10.10.13.3, 02:46:30, POS2/0
                       [90/2174976] via 10.10.12.2, 02:46:30, FastEthernet1/0
      172.10.0.0/32 is subnetted, 5 subnets
D        172.10.0.2 [90/156160] via 10.10.12.2, 02:46:32, FastEthernet1/0
D        172.10.0.3 [90/146944] via 10.10.13.3, 02:46:31, POS2/0
D        172.10.0.4 [90/2297856] via 10.10.14.4, 02:46:35, Serial3/0
D        172.10.0.5 [90/149504] via 10.10.13.3, 02:46:30, POS2/0


 
I hope you have found this article informative and useful and now have a fair understanding of EIGRP Tables, EIGRP Terminology and Operation. For any of the related queries or feedback, kindly write to us at networkurge@gmail.com
 

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